1. Field of the Invention
This invention relates to a method of stimulating a subterranean coal seam in order to increase gas production therefrom. In another aspect, this invention relates to a method of cavitating a coal seam that employs a foam forming liquid and an expandable fluid.
2. Description of the Prior Art
Many subterranean coal seams contain large volumes of trapped hydrocarbon gases including, for example, methane. When economically produced, these gas reserves represent a valuable resource. Once a coalbed well has been drilled and completed, it is common to treat the surrounding coal seam in order to stimulate the gas production therefrom. Generally, stimulation or “workover” procedures involve creating and/or enlarging pathways for the methane gas to travel from within the formation to the wellbore. Presently, two common methods of stimulating methane production from coalbed wells include hydraulic fracturing and “cavity induced stimulation” or cavitation.
Hydraulic fracturing involves introducing a fracturing fluid into the coal seam at a pressure above the fracture pressure of the coal formation. Hydraulically fractured wells are cased throughout the coal seam and the casing is perforated to allow the fracturing fluid to enter the coal seam at an elevated pressure. One concern associated with hydraulic fracturing is the significant amount of damage it causes the natural cleat network in the coal seam surrounding the wellbore, which adversely impacts the production rate of the well. In addition, the coal fines generated as a result of the high pressure fluid injection combine with the fracturing fluid and plug the natural cleats in the surrounding coal seam, which adversely impacts the gas production rate.
Cavitation is another method employed to stimulate gas production from coalbed methane wells. In general, cavitation involves the formation and/or enlargement of a cavity in the near wellbore region of the coal seam. Typically, cavitation is accomplished by allowing fluid pressure to build in the coal seam and then releasing the pressure to fragment a portion of the coal, which creates and/or enlarges a cavity in the coal seam. Cavitation can also increase the permeability of the surrounding formation, which results in greater increases in gas production rates compared to hydraulically stimulated wells. Thus, cavitation is often the preferred method of coalbed stimulation. However, current cavitation methods have limited effectiveness when applied to certain types of coalbed methane wells, especially wells penetrating coal seams having a high permeability and a low reservoir pressure.
Thus, a need exists for an improved method of increasing gas production from a coalbed methane well that minimizes coal seam damage and can be successfully applied to various types of coal seams.